MICA is a ligand for NKG2D, a C-type lectin-like, type II transmembrane receptor expressed on most human NK cells, γδ T cells, and CD8+ T cells. Upon ligation, NKG2D signals through the adaptor protein DAP10 to evoke perforin dependent cytolysis and to provide co-stimulation. In humans, the NKG2D ligands include MHC class I chain-related protein A (MICA), the closely related MICB, UL-16 binding proteins (ULBP) 1-4, and RAE-1G.
While NKG2D ligands are not usually found on healthy tissues, various forms of cellular stress, including DNA damage, may upregulate ligand expression, resulting in their frequent detection in multiple solid and hematologic malignancies, including melanoma. NKG2D activation through ligand positive transformed cells contributes to extrinsic tumor immunity, since NKG2D deficient mice manifest enhanced tumor susceptibility. But in many cancer patients NKG2D-mediated tumor immunity is ineffective. In part, immune escape may be achieved by the shedding of NKG2D ligands from tumor cells, which triggers internalization of surface NKG2D and impaired function of cytotoxic lymphocytes. See e.g., Wu et al., “Prevalent Expression of the Immunostimulatory MHC Class I Chain-related Molecule is Counteracted by Shedding in Prostate Cancer,” J Clin Invest 114: 560-8 (2004); Groh et al., “Tumour-derived Soluble MIC Ligands Impair Expression of NKG2D and T-cell Activation,” Nature 419: 734-8 (2002); Doubrovina et al., “Evasion from NK Cell Immunity by MHC Class I Chain-related Molecules Expressing Colon Adenocarcinoma,” J Immunol 171:6891-9 (2003). A reduction in the density of MIC expressed on the tumor cell surface due to MIC shedding from tumors is also one of the mechanisms for tumor evasion. See Marten et al., “Soluble MIC is Elevated in the Serum of Patients with Pancreatic Carcinoma Diminishing Gamma Delta T Cell Cytotoxicity,” Int J Cancer 119:2359-65 (2006). Soluble NKG2D ligands may also stimulate the expansion of regulatory NKG2D+CD4+Foxp3− T cells that may antagonize anti-tumor cytotoxicity through Fas ligand, IL-10, and TGF-β.
MICA is a NKG2D ligand shed from tumor cells, i.e., released from the cell surface into the surrounding medium, and sera from a subset of cancer patients contains elevated levels of the soluble form (sMICA). MIC (the term “MIC” referring to MICA and MICB) shedding is accomplished in part through interactions with the protein disulfide isomerase ERp5, which cleaves a disulfide bond in the MIC α3 domain, rendering it susceptible to proteolysis by ADAM-10/17 and MMP14. Methods of treating cancer by administering anti-MIC antibodies or antigen-binding peptide fragments have been described. For example, U.S. Pat. No. 8,182,809 describes such methods utilizing a purified antibody or a polypeptide comprising an antigen-binding fragment thereof that specifically binds to the amino acid sequence NGTYQT located in the α3 ectodomain of the MIC polypeptide, such that the interaction of the MIC polypeptide and ERp5 is inhibited and the shedding of MIC is inhibited. And U.S. Pat. No. 7,959,916 describes methods of inhibiting the shedding of MIC polypeptides from cancer cells using anti-MIC α3 domain antibodies. Tumor-derived soluble MIC polypeptides, either MICA or MICB, or both, have also been suggested as biomarkers for diagnosis and prognosis of cancer and anti-MICA or anti-MICB antibodies as therapeutic agents for the treatment of cancer and autoimmune diseases. For example, U.S. Pat. No. 7,771,718 describes methods of relieving MIC-induced suppression of NKG2D in lymphocytes using anti-MIC antibodies to bind soluble MIC polypeptides.
In practice, methods of treating cancer or other diseases with therapeutic antibodies is relatively expensive because of the need to produce large quantities of such antibodies of sufficient purity for infusion to patients. In view of the complexity of large-scale antibody production and the specialized requirements for antibody infusion protocols, alternative methods are needed to target MIC polypeptides in a more efficient and cost-effective manner. The present invention provides a solution to this problem by providing vaccines for the induction of anti-MIC antibodies in a subject.
Tumor vaccines are typically composed of tumor antigens and immunostimulatory molecules (e.g. cytokines or TLR ligands) that work together to induce antigen-specific cytotoxic T cells (CTLs) that recognize and lyse tumor cells. At this time, almost all vaccines contain either shared tumor antigens or whole tumor cell preparations (Gilboa, 1999). The shared tumor antigens are immunogenic proteins with selective expression in tumors across many individuals and are commonly delivered to patients as synthetic peptides or recombinant proteins (Boon et al., 2006). In contrast, whole tumor cell preparations are delivered to patients as autologous irradiated cells, cell lysates, cell fusions, heat-shock protein preparations or total mRNA (Parmiani et al., 2007). Since whole tumor cells are isolated from the patient, the cells express patient-specific tumor antigens as well as shared tumor antigens. Finally, there is a third class of tumor antigens that has rarely been used in vaccines due to technical difficulties in identifying them (Sensi et al. 2006). This class consists of proteins with tumor-specific mutations that result in altered amino acid sequences. Such mutated proteins have the potential to: (a) uniquely mark a tumor (relative to non-tumor cells) for recognition and destruction by the immune system (Lennerz et al., 2005); (b) avoid central and sometimes peripheral T cell tolerance, and thus be recognized by more effective, high avidity T cells receptors (Gotter et al., 2004).